The escalating requirements for high-density and performance associated with ultra large-scale integration semiconductor wiring require responsive changes in interconnection technology. Such escalating requirements have been found difficult to satisfy in terms of providing a low RC (resistance capacitance) interconnection pattern, particularly where sub-micron via contacts and trenches have high aspect ratios imposed by miniaturization.
Conventional semiconductor devices typically comprise a semiconductor substrate, normally of doped monocrystalline silicon, and a plurality of sequentially formed dielectric layers and conductive patterns. An integrated circuit is formed containing a plurality of conductive patterns comprising conductive lines separated by interwiring spacings, and a plurality of interconnect lines, such as bus lines, bit lines, word lines and logic interconnect lines. Typically, the conductive patterns on different layers, i.e., upper and lower layers, are electrically connected by a conductive plug filling a via hole, while a conductive plug filling a contact hole establishes electrical contact with an active region on a semiconductor substrate, such as a source/drain region. Conductive lines are formed in trenches which typically extend substantially horizontal with respect to the semiconductor substrate. Semiconductor "chips" comprising 5 or more levels of metalization are becoming more prevalent as device geometries shrink to submicron levels.
A conductive plug filling a via hole is typically formed by depositing a dielectric interlayer on a conductive layer comprising at least one conductive pattern, forming an opening in the dielectric interlayer by conventional photolithographic and etching techniques, and filling the opening with a conductive material, such as tungsten (W). Excess conductive material on the surface of the dielectric layer is typically removed by chemical mechanical polishing (CMP). One such method is known as damascene and basically involves forming an opening in the dielectric interlayer and filling the opening with a metal. Dual damascene techniques involve forming an opening comprising a lower contact or via hole section in communication with an upper trench section, which opening is filled with a conductive material, typically a metal, to simultaneously form a conductive plug and electrical contact with a conductive line.
High performance microprocessor applications require rapid speed of semiconductor circuitry. The control speed of semiconductor circuitry varies inversely with the resistance and capacitance of the interconnect pattern. As integrated circuits become more complex and feature sizes and spacings become smaller, the integrated circuit speed becomes less dependent upon the transistor itself and more dependent upon the interconnection pattern. Miniaturization demands long interconnects having small contacts and small cross-sections. Thus, the interconnection pattern limits the speed of the integrated circuit. If the interconnection node is routed over a considerable distance, e.g., hundreds of microns or more as in sub-micron technologies, the interconnection capacitance limits the circuit node capacitance loading, and, hence, the circuit speed. As integration density increases and feature size decreases in accordance with sub-micron design rules, e.g., a design rule of about 0.18.mu. and below, the rejection rate due to integrated circuit speed delays severely limits production throughput and significantly increases manufacturing costs.
In prior technologies, aluminum was used in very large scale integration interconnect metalizations. Copper and copper alloys have received considerable attention as a candidate for replacing aluminum in these metalizations. Copper has a lower resistivity than aluminum and improved electrical properties vis-a-vis tungsten, making copper a desirable metal for use as a conductive plug as well as conductive wiring.
In the formation of a dual damascene structure in a self-aligned manner, a conductive line and vias that connect the line to conductive elements in a previously formed underlying conductive interconnect layer, are simultaneously deposited. The conductive material is deposited into openings (e.g., via holes and trenches) created in dielectric material that overlays the conductive interconnect layer. Typically, a first layer of dielectric material is deposited over a bottom etch stop layer that covers and protects the conductive interconnect layer. A middle etch stop layer is then deposited over the first dielectric layer. A pattern is then etched into the middle stop layer to define the feature, such as a via hole, that will later be etched into the first dielectric layer. Once the middle etch stop layer is patterned, a second dielectric layer is deposited on the middle etch stop layer. A hard mask layer may then be deposited on the second dielectric layer. A desired feature, such as a trench, is etched through the hard mask layer and the second dielectric layer. This etching continues so that the first dielectric layer is etched in the same step as the second dielectric layer. The etching of the two dielectric layers in a single etching step reduces the number of manufacturing steps. The bottom etch stop layer within the via hole, which has protected the conductive material in the conductive interconnect layer, is then removed with a different etchant chemistry. With the via holes now formed in the first dielectric layer and a trench formed in the second dielectric layer, conductive material is simultaneously deposited in the via and the trench in a single deposition step. (If copper is used as the conductive material, a barrier layer is conventionally deposited first, to prevent copper diffusion.) The conductive material makes electrically conductive contact with the conductive material in the underlying conductive interconnect layer.
In efforts to improve the operating performance of a chip, low k dielectric materials have been increasingly investigated for use as replacements for dielectric materials with higher k values. Lowering the overall k values of the dielectric layers employed in the metal interconnect layers lowers the RC of the chip and improves its performance. However, low k materials, such as benzocyclobutene (BCB), hydrogen silsesquioxane (HSQ), SiOF, and the material sold under the trade name of FLARE, are often more difficult to handle than traditionally employed higher k materials, such as an oxide.
When forming a dual damascene structure in a self-aligned manner in which a low k dielectric material, such as BCB, is substituted for higher k dielectric materials in the two dielectric layers in which the vias and the trench are created, the problem of "undercutting" becomes a concern. Undercutting is the undesired enlargement of one of the holes created in one of the dielectric layers. As seen in FIG. 1 which depicts a cross-section of an interconnect region processed in accordance with the prior art, a trench dielectric layer 16 is overetched during the etching of the trench dielectric layer 16 and a via dielectric layer 12. (The middle etch stop layer 14 was previously etched prior to the deposition of the second dielectric layer 16.) An undercut appears below a hard mask layer 18 and is due to the overetching. The overetching typically occurs to assure adequate etching of the via dielectric layer 12 down to a bottom etch stop layer 12 that overlays the conductive layer 10. The undercutting causes the ultimately formed via structure to be incorrectly shaped and presents a critical dimension control problem in a low k etching process.
There is a need for a method and arrangement that provides a film with a lower overall dielectric constant value that will exhibit improved overall performance, yet avoids the undercutting that occurs with the use of a low k dielectric material in the dielectric layers of a dual damascene interconnect arrangement.